Fluoride ion fluorinates the hydroxyapatite of dentin and thus strengthen the dentin. Therefore, fluoride ion has much to be expected of for use in inhibiting or preventing dental caries. Further, it is conceivable to use fluoride ions in combination with calcium ion and phosphoric ion for capping dental tuble, calcification thereof, and/or recalcification of softened dentin, through which much can be expected for protection of dental pulp and otherwise.
Hitherto, in the field of dentistry, for the purpose of preventing dental caries, as well as inhibiting secondary dental caries, there have been used fluoride-ion releasable compounds, such as sodium fluoride, potassium fluoride, aluminum fluoride, sodium monofluorophosphate, strontium fluoride, zinc fluoride, primary tin fluoride, and fluorides of rare earth elements. Dental compositions containing such compounds are also known.
Primarily, however, liberation of fluoride ions is a dissociation of the fluoride ions in the presence of water and often involves an elution, i.e., dissolution, of a compound, which in effect means a disintegration of a composition and, in addition, the presence of counter ions. Therefore, the above noted known compounds are unstable for use in dental compositions.
In the mean while, cements formed by the reaction of glass with an ionomer are well known as a dental composition which is releasable fluoride ion, and have many uses in dentistry, for example, the restoration of cavities or lesions, sealing or filling occlusal pits and fissures, sealing root surfaces for overdentures or for luting.
These ionomer cements are hydrogel salts that are formed by the reaction between a basic calcium aluminofluorosilicate glass and an acidic polyelectrolyte which is a homopolymer or a copolymer of unsaturated carboxylic acid. Cements of this type are particularly preferred as dental cements because they are highly biocompatible, bond strongly to the tooth structure and allow fluoride to be released. However, the setting reaction between the glass and the ionomer is slow and therefore the length of time required for the cement to set to a sufficient hardness for the dentist to perform the necessary finishing presents a problem.
Dental cements have also been proposed which contain a filler and a light-curable resin, so that the setting of the cement is effected by its exposure to light. In one embodiment the filler is glass and the resin is chosen to be an ionomer which reacts with the glass and is also light-curable. The time that is required for the cement to reach the required hardness is thereby reduced. This improvement has provided significant improvements over the traditional glass ionomer cements. However, these light-curable compositions have the disadvantages that only a limited range of monomers can be used. This is because they must be both ionomeric and light-curable.
Moreover, in order that the dentist can use the cement, it is essential that it does not set until after it has been placed in the tooth. The glass and the ionomer are therefore commonly supplied separately in the form of a powder and a liquid respectively. The dentist must then mix them in the appropriate ratio immediately prior to application to the tooth. As the setting process commences on contact between the ionomer and the glass, the dentist must work quickly to mix the cement and to apply it to the tooth. The cement is then set by exposing to a light. In general, setting is accelerated by exposing the cement to a bright light.
The drawbacks of this method are that it is difficult to get the ratio between the components correct and to achieve efficient mixing. These drawbacks result in variations in the composition of the final cement.
There is also a danger that during mixing air bubbles will be introduced into the mixture which will lead to weakness in the final cement. A further difficulty in achieving a product having the correct ratio of glass to ionomer is that the components are highly temperature and humidity sensitive and results may therefore vary from day to day.
In an attempt to overcome these drawbacks, the powder and liquid are sometimes provided in a single capsule where they are separated by a membrane. Immediately prior to use, the capsule is placed in a machine which pierces the membrane and vibrates the capsule to mix the components. The mixture is then applied to the tooth and cured by exposure to light. Whilst this procedure may overcome the mixing difficulties of the two-pack system described above, the cement provided in the capsule still has the drawback that setting commences immediately the components are mixed.
Accordingly, it is an object of the invention to provide a filler composition which eliminates aforesaid problems, and which is capable of stable and sustained fluoride ion release without involving elution and is not liable to disintegrate a compound. It is another object of the invention to provide dental compositions containing the filler and, more particularly, a light curable one-pack type dental cement.